Second RAM Stick Black Screen

I have two identical DDR4 16GB 3200MHz RAM sticks (a matched pair).

The first stick works perfectly by itself. The second stick does not work:

• If I install only the second stick, the PC powers on but I get a black screen.
• If I install both sticks together, I also get a black screen.
• If I remove the second stick and keep only the first one, the PC boots normally and shows 16GB.

Important details:

• Both RAM sticks worked before on my old motherboard.
• Later, my old motherboard developed problems after I changed some BIOS settings and Secure Boot settings.
• I replaced the motherboard with a new one.
• The repair technician tested the new motherboard using only the first RAM stick, and the PC worked.
• When I came home and installed the second RAM stick, the problem started.
• I tested the second RAM stick alone in the same slot where the first one works, and it still gave a black screen.
• I also tried different RAM slots and cleaned the gold contacts carefully.

The second RAM stick was stored in my wallet for some time. It may have been exposed to dust, but it was not dropped, bent, exposed to water, or hit.

Visually, the RAM looks completely normal. No burned components, no corrosion, no missing chips, and no physical damage.

Could dust or poor contact really cause this, or is the second RAM stick permanently defective?